#define _CRT_SECURE_NO_WARNINGS
class Solution {
    int tmp[50001];
public:
    int reversePairs(vector<int>& nums)
    {
        return mergesort(nums, 0, nums.size() - 1);
    }
    int mergesort(vector<int>& nums, int left, int right)
    {
        if (left >= right)
        {
            return 0;
        }
        int ret = 0;
        int mid = left + (right - left) / 2;

        ret += mergesort(nums, left, mid);
        ret += mergesort(nums, mid + 1, right);

        int cur1 = left, cur2 = mid + 1, i = 0;

        while (cur1 <= mid)
        {
            while (cur2 <= right && nums[cur2] >= nums[cur1] / 2.0)cur2++;
            if (cur2 > right)
            {
                break;
            }
            ret += right - cur2 + 1;
            cur1++;
        }
        cur1 = left, cur2 = mid + 1, i = left;
        while (cur1 <= mid && cur2 <= right)
        {
            tmp[i++] = nums[cur1] <= nums[cur2] ? nums[cur2++] : nums[cur1++];
        }
        while (cur1 <= mid)
        {
            tmp[i++] = nums[cur1++];
        }
        while (cur2 <= right)
        {
            tmp[i++] = nums[cur2++];
        }

        for (int j = left; j <= right; j++)
        {
            nums[j] = tmp[j];
        }
        return ret;
    }
};